1. Field of the Invention
The present invention relates to a mobile-station transmitter/receiver, and more particularly, to a mobile-station transmitter/receiver suitable for organization of a mobile communications network system using the code division multiplex access, for example.
2. Description of Related Art
In the conventional mobile communications network systems, the mobile-station transmitter/receiver makes an intermittent reception while waiting for an incoming communication to avoid battery consumption for the standby period. The conventional mobile communications network systems currently use various types of mobile-station transmitter/receivers using frequency division multiplex access (FDMA), time division multiplex access (TDMA), code division multiplex access (CDMA), etc. for intermittent reception in practice. These intermittent reception methods have some advantages and are disadvantageous in some points. For a mobile communications network system, any of the above methods is selected which has suitable characteristics for a purpose of the mobile communications network system. Also, the mobile-station transmitter/receiver adopting an intermittent reception method is defined in detail for each mobile communications network system.
A mobile-sation transmitter/receiver based on the code division multiplex access (CMDA), for example, uses a special code (pseudorandom noise sequence=PN) allocated to each communications circuit to spread a modulated wave derived from a carrier frequency to a wider band than that of the original carrier frequency (will be referred to as xe2x80x9cspectrum spreadingxe2x80x9d hereinunder) and modulates each modulated wave thus spectrum-spread for transmission. By synchronizing a received spectrum-spread signal with a PN code supplied via a circuit for which demodulation is intended, the mobile-station transmitter/receiver discriminates only the intended circuit from others.
More particularly, the transmitting side will allocate different PN codes to individual communications circuits. The PN code comprises a pseudorandom noise sequence as mentioned above. The transmitting side multiplies each modulated wave for transmission via each circuit by a PN code, different from one circuit to another, to spectrum-spread the wave. The modulated wave has been subjected to a predetermined modulation before the spectrum spreading. Each modulated wave thus spectrum-spread is multiplexed for transmission.
At the receiving side, an incoming signal from the transmitting side is multiplied by a same PN code as a one allocated to a circuit for which demodulation is intended, with the PN codes being synchronized with each other. Thus, only the modulated wave having been transmitted via the intended circuit can be demodulated.
As aforementioned, CDMA allows a direct communication to be made between the transmitting and receiving sides at each call only by setting a same PN code at both the sides. With CDMA, since a modulated wave is spectrum-spread using different PN code for each circuit, the receiving side can only demodulate a spectrum-spread signal transmitted via a circuit for which demodulation is intended. Also the PN code is a pseudorandom noise sequence as described in the above. Therefore, the CDMA method can be said to be superior in keeping confidentiality of information transferred between the transmitting and receiving sides.
In a mobile communications network system using CDMA, the base-station transmitter/receiver at the receiving side repeatedly sends a PN code (will be referred to as xe2x80x9cpilot signalxe2x80x9d hereinunder) to acquire and maintain a synchronization at a mobile-station transmitter/receiver and to regenerate a clock. Detecting pilot signals sent from a plurality of base-station transmitters/receivers, the mobile-station transmitter/receiver at the receiving side allocates detected timings to demodulators, respectively. In the mobile-station transmitter/receiver, the demodulator generates PN code. By multiplying, by a generated PN code, a spectrum-spread signal transmitted from an intended base-station transmitter/receiver at an allocated timing, the mobile-station transmitter/receiver demodulates the signal.
For shift from continuous to intermittent mode of reception, the mobile-station transmitter/receiver calculates a start timing of a next slot and goes into non-reception interval. Before the start timing of a desired slot, the mobile-station transmitter/receiver detects a pilot signal sent from the base-station transmitter/receiver, allocates te pilot signal to a demodulator for demodulation of the signal, and gets into a slot again. After receiving a pilot signal in the desired slot, the mobile-station transmitter/receiver calculates a start timing of a next slot and goes into non-reception interval. Thereafter, the mobile-station transmitter/receiver will repeat the above-mentioned procedure to implement an intermittent reception.
Generally, even while the mobile-station transmitter/receiver is waiting for an incoming communication, it is sending a control information to the base-station transmitter/receiver in order to register its location required for standing in the mobile communications network system. When sending the control information to the base-station transmitter/receiver, the mobile-station transmitter/receiver shifts from the intermittent to continuous reception to receive a response from the base-station transmitter/receiver. Namely, while the mobile-station transmitter/receiver is waiting for an incoming communication, it is receiving with frequent shift between the continuous and intermittent modes of reception.
The mobile-station transmitter/receiver shifts from a continuous to intermittent mode of reception following the steps (1) to (7) as shown in FIG. 1.
(1) A request for shift from a continuous to intermittent reception is generated.
(2) Start timing of a slot allocated to a mobile-station transmitter/receiver in the closest proximity is calculated to assure a time of preparation for re-synchronization, namely, to be in time to the slot.
(3) The mobile-station transmitter/receiver is set to get started at the start time of a preparation for the intermittent reception, and stops the reception.
(4) The mobile-station transmitter/receiver gets started at the start time of the preparation for the intermittent reception, and detects a pilot signal.
(5) The mobile-station transmitter/receiver uses the detected pilot signal to start a demodulation.
(6) Upon completion of the slot, the mobile-station transmitter/receiver calculates a start timing of a next slot.
(7) The mobile-station transmitter/receiver is set to get started at the start time of a preparation for intermittent reception, and stops the reception. Thereafter, it repeats the above steps.
Nevertheless, when shifting from a continuous to intermittent reception as mentioned in the above, the mobile-station transmitter/receiver will have to have synchronization with the base-station transmitter/receiver prior to a start timing of slot. In particular, it is necessary in CDMA for the mobile-station transmitter/receiver to search and re-acquire, prior to the start timing of slot, a pilot signal it has once lost because it has been in the non-reception status in order to shift to the intermittent mode of reception and to allocate the re-acquired pilot signal to a demodulator. To this end, the mobile-station transmitter/receiver needs a time to recover from the non-reception status before the start timing of its intended slot and make a preparation for the start of reception.
Start and stop of intermittent reception are determined by a call control protocol. A request for shift between continuous and intermittent modes of reception is generated irregularly. When the mobile-station transmitter/receiver decides to shift from a continuous to intermittent reception, it has to select a start timing of a slot which assures a period of preparation for starting the slot. As shown in FIG. 2, if a period between the start timing of a slot allocated to the mobile-station transmitter/receiver and a requested timing for shift from continuous to intermittent reception is shorter than a predetermined length of time, the mobile-station transmitter/receiver cannot get ready for reception in a next slot.
Thus, if the time of preparation for starting a reception in a nearest slot allocated to the mobile-station transmitter/receiver is insufficient when a request for shift from a continuous to intermittent reception is generated, the mobile-station transmitter/receiver will fail to receive in the allocated slot at the time of shift from continuous to intermittent reception. In this event, the mobile-station transmitter/receiver will not be able to make a reception in the nearest allocated slot and will do it by selecting a start timing of a next slot following the closest allocated slot.
If the period of the slot for an intermittent reception is short, failure to receive in the allocated slot will relatively less influence the performance of the mobile-station transmitter/receiver. If the period of the slot for an intermittent reception is long, however, the failure to receive in the allocated slot will cause such an influence that the mobile-station transmitter/receiver seems as if it had the performance deteriorated. For example, if the mobile-station transmitter/receiver fails to receive in an allocated slot when a request for reception is made, it will have to be in the non-reception status until reception in a next slot, so that the mobile-station transmitter/receiver will not respond to the calling side having requested for the reception for a long time.
It is therefore an object of the present invention to provide a mobile-station transmitter/receiver adapted not to fail in receiving in an allocated slot at the time of frequent shift from continuous to intermittent reception.
According to the present invention, there is provided a mobile-station transmitter/receiver, in which a start timing of a next slot allocated to the mobile-station transmitter/receiver is calculated at the time of a shift from a continuous to intermittent mode of reception; it is compared with a required timing for shift to an intermittent reception; and when the mobile-station transmitter/receiver could not get ready for starting the reception in a nearest slot can be done by the start of the slot, the continuous reception is maintained until completion of the slot to prevent a failure in reception in a slot.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiment of the present invention when taken in conjunction with the accompanying drawings.